The effect of light on survival of entomopathogens is well described and efforts are underway to develop formulations that may protect an entomopathogen from damage by sunlight. The availability of solar simulators allows for year-round testing of solar protectants. A commercial formulation of Bacillus thuringiensis Berliner and an unformulated baculovirus isolated from Anagrapha falcifera (Kirby) were exposed to various amounts of light from a solar simulator or the sun to determine the relative effect of each source on loss of insecticidal activity. Rate of pathogen degradation was essentially the same for both light sources when original activity remaining was regressed against total energy (as measured by joules/m²). The amount of time required to reduce activity was different, however, because of a difference in total energies produced by the solar simulator and natural sunlight. Virus was approximately two times more sensitive to light than bacteria. To obtain 50% reduction of virus activity, exposure to 1.8 × 10⁷ joules was required, whereas 3.2 × 10⁷ joules was necessary to achieve a similar loss of activity for B. thuringiensis. The importance of reporting energy levels from various solar simulators is discussed.